Synthesis, Fluorescence and Photoisomerization Studies of Azobenzene-Functionalized Poly(alkyl aryl ether) Dendrimers

Article abstract of DOI:10.1002/chem.200305297

A series of azobenzene-functionalized poly(alkyl aryl ether) dendrimers have been synthesized and their photochemical and photophysical properties in solution and as thin films have been investigated. Although the photochemical behavior of the azodendrime

Full text of DOI:10.1002/chem.200305297

FULL PAPER
Synthesis, Fluorescence and Photoisomerization Studies of Azobenzene-
Functionalized Poly(alkyl aryl ether) Dendrimers
Jayaraj Nithyanandhan,[a] Narayanaswamy Jayaraman,*[a] Riju Davis,[b] and
Suresh Das*[b]
Abstract: A series of azobenzene-func-
tionalized poly(alkyl aryl ether) den-
drimers have been synthesized and
their photochemical and photophysical
properties in solution and as thin films
have been investigated. Although the
photochemical behavior of the azoden-
drimers in solution indicated that the
azobenzene units behave independent-
ly, very similar to the constituent mon-
omer azobenzene unit, the properties
of thin solid films of the dendrimers
were distinctly different. The azoden-
drimers, AzoG1, AzoG2, and AzoG3
were observed to form stable super-
cooled glasses, which showed long-
wavelength absorption and red emis-
sion characteristics of J-aggregates of
the azobenzene chromophores. Rever-
sible photoinduced isomerization of the
azodendrimers in the glassy state is de-
scribed.
Keywords: aggregation
¥
azo
compounds ¥ dendrimers ¥ glasses ¥
photochemistry
Introduction
creasing attention. With fewexceptions, in most of the re-
ported studies, the photoresponsive behavior of the azo
chromophores is not perturbed by incorporation into the
dendrimer structure and they behave essentially as decou-
pled units. A comparative study of the isomerization behav-
ior of azobenzene units, reported recently by Caminade,
Majoral, and co-workers,[3] had shown that the locations of
azobenzene in a dendritic structure affect the isomerization
rates, with the peripheral azobenzene units undergoing
faster cis-trans isomerization than those located at the interi-
or of the dendrimer structure. In a recent study, De Schryver
and co-workers[4] have observed that azobenzene-functional-
ized poly(propyleneimine) dendrimers undergo structural
inversion in aqueous acidic media, resulting in the formation
of micrometer-sized vesicles. The formation of these vesicles
leads to an ordered arrangement of the azobenzene chromo-
phores resulting in changes in their emission properties. Ir-
radiation of the vesicles with 420 nm light resulted in signifi-
cant changes in the ordering and the emission behavior of
the azo chromophores. Studies by Vˆgtle and Balzani
et al.[5] have shown that dendrimers functionalized with cis-
azobenzenes act as better hosts for guest molecules than do
the corresponding dendrimers containing the trans isomers.
The use of such azodendrimers in holographic optical data
storage has also been explored.[5b] A further important de-
velopment is the demonstration by Advincula and co-work-
ers,[6] that azobenzene-functionalized PAMAM dendrimers
not only phase segregate between the dendrimer region and
the peripheral azobenzene region within a molecule, but
also form aggregates interdigitized with azobenzene units.
With the above and more reports[7] on azobenzene-function-
In the domain of macromolecules, the dendritic architecture
occupies a unique place because it possesses highly regular
branching units and the ability to incorporate exponentially
increasing number of functional moieties in each subsequent
generation.[1] Moreover, unlike conventional macromole-
cules, the spatially uniform distribution of chain ends can
provide a highly dense distribution of relatively disentangled
functional moieties, which can extend up to nanometric
structural dimensions. Recently, there has been a growing
interest in the design of dendrimeric systems in which the
structural features and thereby the properties, such as self-
assembly, liquid crystallinity, and encapsulation, can be con-
trolled by light.[2] In viewof this, the study of dendrimers in
which photochromic molecules such as azobenzene form an
integral part of the structural unit has been attracting in-
[a] J. Nithyanandhan, Dr. N. Jayaraman
Department of Organic Chemistry
Indian Institute of Science
Bangalore 560 012 (India)
Fax : (+91)80-2360-0529
E-mail: jayaraman@orgchem.iisc.ernet.in
[b] Dr. R. Davis, Dr. S. Das
Photosciences and Photonics Division
Regional Research Laboratory (CSIR)
Trivandrum-695 019 (India)
Fax : (+91) 471-2490-186
E-mail: sdaas@rediffmail.com
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
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DOI: 10.1002/chem.200305297
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FULL PAPER
alized dendrimers, it is fairly clear that high density and uni-
form distributions of an azobenzene chromophore on a den-
dritic structure provide an ideal framework for the evalua-
tion of the macroscopic properties in relation to the cluster-
ing of the chromophores within each molecule.
benzene unit for the functionalization of the dendrimers.
The monomer AzoM was synthesized by first coupling 4-bu-
tylaniline with phenol in the presence of HNO2 at 08C, fol-
lowed by reacting the resulting 4-butyl-4’-hydroxyazoben-
zene with 1,5-dibromopentane in the presence of K2CO3.
Hydroxyl group terminated 0 (G0), 1 (G1), 2 (G2), and 3
(G3) generation dendrimers, possessing 3, 6, 12, and 24 pe-
ripheral hydroxyl groups, respectively, were O-alkylated
with AzoM. Multifold O-alkylation of these dendrimers was
performed with a slight excess of AzoM (1.1 molar equiva-
lents for each hydroxy group), in the presence of either
K2CO3 or Cs2CO3. Upon completion of the O-alkylation,
the azobenzene-functionalized dendrimers AzoG0, AzoG1
(Scheme 1), AzoG2 (Scheme 2), and AzoG3 (Scheme 3)
were purified by routine column chromatography (SiO2)
and product identification was possible by TLC analysis for
all the dendrimers. While the O-alkylation yields were good
for zero (AzoG0) and first (AzoG1) generation dendrimers,
only moderate yields could be obtained in for the second
(AzoG2) and third (AzoG3) generation dendrimers. The
constituent monomer AzoM and the zero generation den-
drimer AzoG0 exist as solids, whereas the dendrimers
AzoG1, AzoG2, and AzoG3 are either low-melting solids or
gummy substances at room temperature. All of these com-
pounds are freely soluble in most organic solvents.
In the present work, we report a study of the behavior of
azobenzene-terminated poly(alkyl aryl ether) dendrimers of
0 3 generations, containing 3, 6, 12, and 24 azobenzene
units, respectively, in both solution and in the solid state.
These studies provide clear evidence for the importance of
dendritic structure in endowing anomalous material proper-
ties emanating from the chromophoric unit. The major find-
ings herein include the observation that while all of the den-
drimers exist as monomers in solution, they exist as aggre-
gates and, as a consequence, exhibit fluorescence in thin
solid films. We also find that while the azobenzene mono-
mer is a thermotropic LC in its trans form, the dendrimers
are nonmesogenic. We present a detailed study of the ab-
sorption, emission, and isomerization properties of these
azobenzene-functionalized poly(alkyl aryl ether) dendrimers
in the solution and in the solid state.
Results
Synthesis and characterization: We previously reported[8]
the synthesis of phloroglucinol-based poly(alkyl aryl ether)
dendrimers of up to four generations with either O-benzyl
protected or free hydroxyl group functionalities at their pe-
ripheries. Successful synthesis of these dendrimers involved
repetitive reactions of O-alkylation and O-benzyl group de-
protection of the phloroglucinol moieties in a divergent syn-
thetic fashion. Free hydroxyl group containing dendrimers
were used as pre-formed cores for further modification with
the azobenzene groups. 4-Bromopentyloxy-4’-n-butylazo-
benzene (AzoM) (Scheme 1) served as the monomer azo-
The structural characterization of the azobenzene-func-
1
tionalized dendrimers AzoG0 AzoG3 was carried out by H
and 13C NMR spectroscopy, mass spectrometry, and elemen-
1
tal composition analysis. General patterns of the H NMR
resonances in CDCl3 for all of the dendrimers were: i) sharp
singlets for the inner and peripheral phloroglucinol units; ii)
a set of double doublets of AB type spin system for the azo-
benzene units, and iii) doublets and triplets in the case of
1
pentamethylene and n-butyl segments. The H NMR reso-
nances of AzoG1, AzoG2, and AzoG3 were found to be rel-
atively broad, when compared with AzoG0 in CDCl3. The
1H NMR spectrum of third-gen-
eration dendrimer AzoG3 is
shown in Figure 1. Similarly, 13C
NMR spectra of the dendrimers
AzoG0 AzoG3 exhibited dis-
tinct resonances corresponding
to the C(quat) and CH(me-
thine) of phloroglucinol and
those of the azobenzene units,
as well as resonances for the
pentamethylene and n-butyl
linkers. The 13C chemical shifts
for the azobenzene unit in
AzoM and in the dendrimers
were nearly identical, implying
the absence of any influence on
the 13C resonances owing to the
dendritic architecture as the
generations advance. Complete
functionalization of the periph-
eries of dendrimers AzoG0
AzoG2
was
checked
by
Scheme 1. Molecular structures of the monomer (AzoM), zero (AzoG0) and first (AzoG1) generation den-
drimers.
MALDI-TOF mass spectrome-
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Poly(alkyl aryl ether) Dendrimers
689 698
try, in which the calculated mo-
lecular ion peaks were observed
for each dendrimer. However,
the MALDI-TOF mass spec-
trum of AzoG3 could not be
obtained, despite repeated at-
tempts. Elemental composition
analysis could be performed so
as to confirm the purities of all
the azobenzene functionalized
dendrimers AzoG0 AzoG3.
UV/Vis spectrocopic charac-
terization was performed with
the monomer AzoM and the
dendrimers AzoG0 AzoG3, in
order to evaluate the effect of
clustering azobenzene units
within the dendrimer molecule.
Both the p p* and n p* transi-
tions, at 350 and 450 nm, re-
spectively, originating from the
azobenzene units[9] were found
to remain nearly identical for
all the dendrimers, except that
the molar extinction coefficient
(e) values increase incremental-
ly with each advancing genera-
tion. However, when calculated
Scheme 2. Molecular structure of the second (Azo G2) generation dendrimer.
per azobenzene unit, the
e
values were nearly the same for
all of the dendrimers. This indi-
cates that the trans-azobenzene
chromophores in PhMe solution
are not severely affected by
their attachment at the periph-
eries of the dendrimers. Also, in
the absence of any concentra-
tion-dependent chemical shift
1
changes in the H NMR spectra
in CDCl3, there is no intermo-
lecular association and the den-
drimers remain in their mono-
meric state in solution.
Material properties: Optical po-
larizing microscope studies of
the azobenzene derivatives re-
vealed that only the monomer
AzoM exhibited liquid crystal-
line (LC) properties, which
were characterized by the tex-
tures and sharp phase transi-
tions observed in the heating
and cooling cycles. The azoben-
zene monomer, AzoM exhibit-
ed a smectic A (SA) phase con-
firmed by its focal conic texture
with homeotropic areas (vide
infra). This was confirmed fur-
Scheme 3. Molecular structures of the third (AzoG3) generation dendrimer.
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N. Jayaraman, S. Das et al.
FULL PAPER
Figure 1. 1H NMR spectrum of trans-AzoG3 in CDCl3 (300 MHz).
ther using differential scanning calorimetry (DSC)
(Figure 2). Table 1 summarizes the phase transition tempera-
tures and thermodynamic parameters of AzoM.
Solid-state absorption and emission: The absorption spec-
trum of a thin solid film of AzoM, formed between quartz
plates by cooling from its LC state, possesses a weak absorp-
tion in the 400 500 nm region and a flat, more intense ab-
sorption at l<400 nm.
The absorption spectral features of the azodendrimers
were distinctly different from that of AzoM. The glassy
films of AzoG1, AzoG2, and AzoG3, had absorption spectra
with two clear bands with maxima centered on 355 and
440 nm (Figure 3). The relative intensity of the long-wave-
length band is higher for the azodendrimers than that of
AzoM. The absorption spectrum of AzoG0 was found to be
temperature dependent. Its absorption spectral changes
were monitored as a function of time as it cooled from its
freshly obtained melt (Figure 4). AzoG0 solidifies into a
glassy state from its melt and the absorption spectrum of
this state is shown in Figure 4, curve a. On cooling to room
temperature, the film crystallizes and the absorption spec-
trum of this film is shown in Figure 4, curve b. The ratio of
the absorbance at 350 nm to that at 450 nm increases from
1.2 to 1.8 on going from curve a to curve b.
Figure 2. DSC trace of AzoM with the phase transitions indicated.
The absorption spectra of azobenzene derivatives in solu-
tion are known to have two bands with maxima centered
Table 1. Phase transition temperatures and thermodynamic parameters
of AzoM.
Compound Phase transition temperatures DH
DS
[8C][a]
[kJmolÀ1
]
[JKÀ1 molÀ1
]
AzoM
K 53.3 SA 72.2 I (heating)
I 69.5 SA 42.3 K (cooling)
20.1, 4.9
3.4, 18.7
61.5, 14.2
9.9, 59.3
[a] K = crystalline, SA = smectic A, I = isotropic.
In contrast, none of the dendrimers were found to be
liquid crystalline. Upon heating, dendrimers AzoG1,
AzoG2, and AzoG3 melt to their isotropic states, which do
not crystallize upon cooling, but rather solidify to form su-
percooled, transparent glassy films that were stable at room
temperature for long periods (>6 months). AzoG0, howev-
er, crystallizes upon cooling from its isotropic state. Junge
and McGrath have also reported formation of glasses in
higher order azobenzene-functionalized dendrimers.[7a]
Figure 3. Absorption spectra of thin films of the azobenzene derivatives;
a) AzoG1, b) AzoG3, c) AzoG2 and d) AzoM.
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Poly(alkyl aryl ether) Dendrimers
689 698
Figure 4. Absorption spectra of thin films of AzG0 a) recorded immedi-
ately after melting, b) after 2 min.
Figure 5. Solid state fluorescence spectra of a glassy film of AzoG2 upon
excitation using 450 nm light.
around 350 and 450 nm, attributed respectively to p p* and
n p* transitions. In the present study, the absorption of the
glassy films has a broad band centered on 350 nm and the
relative intensity of the absorption band in the 450 nm
region is much higher than in solution. In bulk materials,
the tendency to form aggregates is very high. Interaction be-
tween chromophores in their ground state on aggregation
has been fairly well explained by McRae and Kasha in
terms of exciton coupling theory.[10] The excited state of ag-
gregates is split into two energy levels (Davydov splitting).
The transition to the upper state is allowed in the case of H-
aggregates, and is characterized by a hypsochromically shift-
ed absorption band, the transition to the lower state is al-
lowed for J-aggregates, and is marked by a bathochromically
shifted absorption band compared with the isolated mono-
mer. The higher intensity of the 450 nm band in the absorp-
tion spectra of the solid films of AzoG1, AzoG2, and
AzoG3, compared with that in solution indicates that there
could be a significant contribution to the absorption in this
region by the red-shifted band associated with J-type aggre-
gates. In AzoG0, the relative intensity of the 450 nm band
compared with that of the 350 nm band is higher in the
glassy state obtained from its melt than in that observed for
its stable crystalline state, indicating that J-aggregate forma-
tion is enhanced in the glassy state. The broadening of the
350 nm band in the glassy films is also indicative of the for-
mation of H-type aggregates.
The excitation spectrum of a glassy film of AzoG2, re-
corded while monitoring emission intensity at 650 nm, has
two maxima at about 410 nm and about 470 nm (see Sup-
porting Information). The presence of two peaks (420 and
525 nm) in the excitation spectra of azobenzene-containing
bilayer membranes reported by Shimomura and Kunita-
ke,[11b] has been attributed to p p* and n p* transitions of
the J-aggregates, respectively.
Solid films of AzoG0 were found to be weakly fluorescent
at room temperature in comparison to their higher order
azodendrimers. As mentioned earlier, the absorption spec-
trum of AzoG0 showed that the formation of J-aggregates is
more pronounced at temperatures close to its melting point.
The fluorescence intensity of AzoG0 when maintained at
temperatures close to its melting point (978C) or in its iso-
tropic state was found to be about four times higher than
that at room temperature. Figure 6 shows the fluorescence
spectra of AzoG0 at room temperature and at 1508C. The
Formation of H- and J-type aggregates has been reported
in some azobenzene-containing bilayers,[11] lipids and vesi-
cles,[4,12] Langmuir Blodgett (LB) films,[13] and liquid crystal-
line phases.[14] Whereas the monomeric form of azobenzenes
is known to be nonfluorescent, their J-aggregates are report-
ed to fluoresce in the 600 nm region.[14b] The solid state fluo-
rescence of these dendrimers was recorded at room temper-
ature. Whereas the solid film of AzoM was nonfluorescent,
the azodendrimers AzoG1 AzoG3 were found to be fluo-
rescent with fluorescence maxima centered at 650 nm.
Figure 5 shows the fluorescence spectra of AzoG2 upon ex-
citation using 450 nm light. Azobenzene derivatives are
known to be nonfluorescent in solutions owing to their abili-
ty to undergo trans cis photoisomerization from the excited
state and also because of the n p* character of the first ex-
cited singlet state.
Figure 6. Solid state fluorescence spectra of AzoG0 a) solid film at room
temperature, b) isotropic state maintained at 1508C; excitation: 450 nm.
fluorescence maximum of the J-aggregate of AzoG0 (lmax
=
~550 nm) was hypsochromically shifted compared with that
of the higher-order dendrimers. AzoM also shows a higher
fluorescence in its isotropic and liquid crystalline phases
than at room temperature in its crystalline state.
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N. Jayaraman, S. Das et al.
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Photochemical properties:
Ea values, which lie in the range of 20 22 kcalmolÀ1, sug-
gests an inversion mechanism for the cis trans isomeriza-
tion. Further, the activation entropies for the dendrimers
are within the range close to AzoM, and these entropic
values indicate that the local environments and the transi-
tion state geometries are similar for the azobenzene chro-
mophores, regardless of their attachment to the dendritic
core.
The above observations agree well with reports on various
other non-push-pull type azobenzene derivatives.[15] These
solution studies also further substantiate previous observa-
tions[5] that the azobenzene units of azobenzene-functional-
ized dendrimers behave very similarly to the corresponding
monomer with no steric or topological constraints.
Photoisomerization in solution: The facile trans cis photo-
isomerization and the reverse cis trans thermal or photoiso-
merization are attractive features of the azobenzene chro-
mophore in general. Isomerization of the azobenzene deriv-
atives was followed by UV/Vis spectroscopy. Upon irradia-
tion (300 360 nm) of a solution of each dendrimer in PhMe
(1.76 39.0mm), a drastic decrease in the absorbance of the
p p* transition and a slight increase in the absorbance of
the n p* transition of the azobenzene unit were observed.
A photostationary state (PSS) equilibrium mixture was at-
tained within about 5 min of the irradiations. On the basis
of the absorbance changes, the PSS mixture was calculated
to contain 85 92% of the cis isomer for each dendrimer.
The thermal reversal of cis trans isomeric form of the azo-
benzene unit was followed in the dark, so as to evaluate the
rate constants and the activation energies associated with
the isomerization process. The kinetics of thermal cis trans
isomerization were followed by time-course measurement of
the absorbance changes at 350 nm for the photoirradiated
solutions of the azobenzene derivatives at different tempera-
tures. The isomerization followed first order kinetics in all
cases as is commonly observed for azobenzene derivatives.
The rate constants and the activation energies determined
for this isomerization process are provided in Table 2. The
Photoisomerization of AzoM in its liquid crystalline phase:
The trans cis photoisomerizations of chromophores have
been extensively utilized in bringing about photoinduced
isothermal phase transitions in liquid crystals.[16] In liquid
crystalline materials containing azobenzene chromophores
as dopants or as an inherent part of the molecular structure,
photoisomerization of the linear trans isomers to the bent
cis isomers is known to disrupt long-range ordering, which
leads to a breakdown of the liquid crystalline phases.[16a]
Photolysis (using 360 nm band-pass filtered output of a
200 W high-pressure mercury lamp) of thin liquid crystalline
films (SA) of AzoM, obtained by slowcooling of its isotropic
state, maintained at 608C, resulted in an isothermal liquid
crystalline to isotropic phase transition. These changes were
thermally reversible. Complete thermal recovery of the ini-
tial SA phase from the isotropic state was observed at 608C
within 8 minutes. Within about 3 min of photolysis, domains
showing the SA texture begin to appear. The optical micro-
graphs recorded at the various stages of the isothermal
phase transitions of AzoM are shown in Figure 7.
Table 2. Rate constants (k) and activation energies (Ea) for thermal cis
trans isomerization.[a]
Compound
Rate constant k [sÀ1
508C 608C
]
Ea
at 408C
708C
AzoM
9.68î10À5
9.56î10À5
8.03î10À5
8.22î10À5
7.92î10À5
2.72î10À4
2.54î10À4
2.50î10À4
2.53î10À4
2.54î10À4
7.17î10À4
7.56î10À4
7.30î10À4
6.93î10À4
7.52î10À4
1.98î10À3
1.64î10À3
1.83î10À3
1.73î10À3
1.57î10À3
21.84
20.98
22.59
21.66
21.97
AzoG0
AzoG1
AzoG2
AzoG3
[a] k is expressed in sÀ1 and Ea in kcalmolÀ1. Ea values were obtained
from the Arrhenius plot.
rate constants at different temperatures for the dendrimers
were nearly the same as that of AzoM. Similarly, the activa-
tion energies (Ea) for the dendrimer series compared well
with AzoM. The pre-exponential factors (A), entropy and
free energy changes associated with the cis-trans isomeriza-
tion, and the values thus obtained from the Arrhenius plots
are shown in Table 3. The log A value of 10 11 for the den-
drimers and the monomer implies a single mechanism for
the isomerization in all cases. This, in combination with the
Table 3. Activation parameters for the cis trans isomerization in PhMe at
408C.
Compound Ea
[kcalmolÀ1
log DG
DS
DH
A
]
[sÀ1] [kcalmolÀ1
]
[calmolÀ1 KÀ1
]
[kcalmolÀ1
]
AzoM
21.84
20.98
22.59
21.66
21.97
11.24 18.33
10.63 16.61
11.67 19.72
11.04 17.87
11.24 18.47
À9.21
À11.99
À7.19
21.22
20.36
21.97
21.04
À9.20
Figure 7. Polarized optical micrographs (400î) depicting the photochemi-
cal phase transition of AzoM: a) SA phase at 608C before photolysis,
b) isotropic phase at 608C upon photolysis using 360 nm light, c) homeo-
tropic SA texture formed in 3 min after formation of the isotropic phase,
d) focal conic SA texture formed in 8 minutes.
AzoG0
AzoG1
AzoG2
AzoG3
À10.12
À9.20
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Poly(alkyl aryl ether) Dendrimers
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Photoisomerization of azodendrimers in their glassy state:
Photolysis of glassy films of the azodendrimers using 360 nm
band-pass filtered output of a 200 W high-pressure mercury
lamp did not bring about any transformation in their absorp-
tion spectra. However significant changes in their absorption
spectra were observed on photolysis using the third harmon-
ic (355 nm) of a Nd:YAG pulsed laser (5 mJ per pulse, 10 ns
pulse width). Figure 8A shows the effect of 355 nm laser
and an increase in intensity of the band around 450 nm,
which is attributable to trans cis photoisomerization. In the
glassy film however the decrease in the 350 nm band is ac-
companied by a shift in the absorption maximum to 320 nm.
A similar shift in absorption maximum was observed on
photolysis of azobenzene-containing dendritic mono-
layers.[13g]
Heating the photolyzed sample above its glass transition
temperature (Tg) results in rapid cis trans isomerization.
This process of generating the cis azobenzene photochemi-
cally and thermal conversion to its trans isomer could be
carried out over several cycles. Similar behavior was ob-
served in the glassy films of AzoG1 and AzoG3.
Discussion
Azobenzene-functionalized dendrimers have been previous-
ly synthesized and studied in several instances. The most
studied dendritic structures functionalized with azobenzene
units at their peripheries were those derived from
1) PAMAM,[6] 2) poly(propylene imine),[4,5] and 3) polysi-
lane dendrimers.[7e,f] Apart from these, azobenzene units
were also incorporated at the interiors of dendritic structure
c]
such as poly(benzyl aryl ether),[7a polyphosphinated,[3] and
polyphenylene dendrimers.[7g] Solution-phase studies reveal
that the properties of the azobenzene unit, especially relat-
ing to isomerization, are highly dependent on the constitu-
ent dendrimer and the location of the azobenzene unit
within the dendrimer. For example, the photoresponsive be-
havior of azobenzene-cored polyphenylene dendrimers is
highly dependent on the dendritic structures,[7g] whereas
such a dependence is negligible in the case of poly(benzyl
aryl ether) dendrimers containing the azobenzene unit at
the core.[7a] One of our initial goals in preparing the poly-
(alkyl aryl ether) dendrimers described herein was to study
the role of dendritic structure, which changes periodically as
a result of consecutive growth of the so-called generations,
on the properties of azobenzene in the solid state and in the
solution phase. Relatively little is known about the photo-
physical and photochemical properties azobenzene-function-
alized dendrimers in the solid state. Recently Vˆgtle, Balza-
ni and co-workers have shown that holographic gratings
with a diffraction efficiency of 20% could be optically re-
corded in thin films of azobenzene-functionalized dendri-
mers.[5b] The existence of dendritic molecules as aggregates
in the solid state, as evidenced from the pronounced hypso-
chromic shift of lmax, was previously shown by Advincula
and co-workers[6] in the case of azobenzene functionalized
PAMAM dendrimers. Similar observations were also report-
ed by Shibaev and co-workers[7e,f] for a azobenzene-function-
alized first generation carbosilane dendrimer. In the present
study a systematic investigation of poly(alkyl aryl ether)
dendrimers AzoG1 AzoG3 showthat thin solid films of
these dendrimers exhibit enhanced absorption in the 450 nm
region when compared with AzoM and AzoG0. This is indi-
cative of the presence of J-type aggregates in the glassy
films of the dendrimers. Additionally, AzoG0 exhibits en-
hanced absorption in the 450 nm region in its glassy state,
Figure 8. Absorption spectra of thin films of AzG2; A) a) glassy form;
b) after photolysis using 355 nm laser 5 mJ per pulse for 30 s, B) temporal
change cis trans isomerization; c) 1.6 h; d) 3.3 h; e) 5.0 h; f) 8.2 h; g)
18.2 h; h) 74.4 h.
photolysis of a glassy film of AzoG2 for 30 s. A decrease in
absorbance of the 350 nm band and an increase in intensity
of the absorption band centered at 450 nm are clearly ob-
servable. The final spectrum obtained (Figure 8A, curve b)
has an absorption maximum that is blue-shifted in compari-
son to that of the unphotolyzed film. At room temperature
(278C), the absorption spectrum of the photolyzed film
shows a slow increase in its absorbance at 350 nm and a con-
comitant decrease in the absorbance at 450 nm; this trans-
formation is marked by the presence of isosbestic points at
320 and 420 nm. The changes in the absorption spectra on
photolysis of the glassy films can be attributed to trans cis
photoisomerization of the azobenzene moieties followed by
a slow, thermal cis trans isomerization. The thermal cis
trans isomerization occurs at a very slowrate and complete
recovery is not observed even after 74 h (curve h, Fig-
ure 8B). The presence of isosbestic points is indicative of a
single process cis trans photoisomerization of azobenzene
groups in thin glassy films.
In solution, photolysis of the azo dendrimers using 350 nm
light results in a decrease in intensity of the 350 nm band
695
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¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

N. Jayaraman, S. Das et al.
FULL PAPER
which is formed at temperatures near its melting point, indi-
cating the formation of J-type aggregates. The correspond-
ing solution-phase studies clearly indicate that all the com-
pounds behave as monomers and the azobenzene units are
essentially decoupled from each other. A consequence of
the formation of the J-aggregate is the observed fluores-
cence in the glassy films of the dendrimers. Such a fluores-
cence property of the J-aggregate of azobenzene is reported
for aqueous bilayer membranes[11a,b] and azobenzene liquid
crystals.[14b] Thus, the fluorescence emission centered at
650 nm observed upon excitation of the thin solid films of
AzoG1 AzoG3 at 450 nm is attributed to the p p* transi-
tion of the lower S2 level, as rationalized by Kunitake and
Shimomura for their bilayer membranes.[11b] With reference
to azobenzene-containing dendrimers, the only known
report on fluorescence involves the azobenzene-functional-
ized poly(propylene imine) dendrimer-derived vesicles.[4] In
contrast to these reports, the present study describes the
first observation of azobenzene fluorescence exhibited by
virgin solid samples of the azobenzene-functionalized den-
drimers. In addtion to the room-temperature fluorescence of
the dendrimers, we observe that AzoM can also be consider-
ably fluorescent at temperatures closer to its melting point.
While AzoM exhibits SmA phase at elevated temperatures,
this smectic phase is also fluorescent, as long as the trans ge-
ometry of the azobenzene unit is retained. Upon photoiso-
merization, conversion to the cis geometry disrupts the chro-
mophore orientation, resulting in the loss of mesogenicity.
While the fluorescence emission of AzoG1 AzoG3 at
room temperature and AzoG0 at elevated temperature is a
consequence of the J-aggregate formation, the photoisome-
rization studies in thin solid films indicate that there also
exist aggregates of the H-type, as evidenced from the ap-
pearance of the 320 nm main absorption band in the photo-
lyzed films, as opposed to the 350 nm band in the unphoto-
lyzed films. These H-type aggregates do not undergo facile
trans cis photoisomerization in the glassy state and thus the
absorption becomes more pronounced as a result of deple-
tion of the trans isomers of the monomer and J-aggregate
on photolysis. Weener and Meijer[13g] have observed that
photoisomerization of azobenzene is restricted in dendritic
monolayers with a strong tendency to form H-aggregates.
isotropic phases, provided it is maintained in its trans
geometry.
In line with a number of previous reports, the solution-
phase properties are unexceptional in that the azobenzene
units are independent of one another in spite of their clus-
tering at the peripheries of dendrimers.
Experimental Section
General methods: Chemicals were purchased from commercial sources
and were used without further purification. K2CO3 (AR grade) was dried
at 120oC for 24 h before being used. Solvents were dried and distilled ac-
cording to literature procedures.[17] Analytical TLC was performed on
commercial Merck plates coated with silica gel GF254 (0.25 mm). Silica
gel (100 200 mesh) was used for column chromatography. Melting points
are uncorrected. Gel permeation chromatography (GPC) was carried out
on a Phenogel (500 ä) semipreparative column (300î7.80 mm) attached
to a high-performance liquid chromatography system fitted with a differ-
ential refractive index (RI) detector; THF was used as the eluent.
Matrix-assisted laser desorption ionization-time-of-flight mass spectrome-
try (MALDI-TOF MS) was performed using either gentisic acid or trans-
indole acrylic acid as the matrix on a Kratos instrument. Microanalyses
were performed on an automated Carlo Erba C, H, N analyzer. 1H and
13C NMR spectral analyses were performed on a 300 MHz and 75 MHz
Jeol spectrometer, respectively, with residual solvent signal acting as the
internal standard. The following abbreviations were used to explain the
multiplicities: s, singlet; d, doublet; t, triplet; m, multiplet; band, several
overlapping signals; br, broad.
AzoM: K2CO3 (0.54 g, 3.94 mmol) and 18-C-6 (cat.) were added to a stir-
red solution of 1,5-dibromopentane (2.71 g, 11.8 mmol) in Me2CO
(20 mL), 4-[(4-butylphenyl)azo]phenol (1.0 g, 3.94 mmol), and the mix-
ture was refluxed for 7 h. The solvents were removed in vacuo and the
resulting residue was dissolved in CH2Cl2, washed with water and brine,
and dried (Na2SO4), concentrated, and purified (SiO2, hexane/EtOAc
95:5) to afford AzoM as an orange solid (1.43 g, 90%). Rf
= 0.43
(hexane/EtOAc 95:5); m.p. 68 708C; 1H NMR (300 MHz, CDCl3): d =
0.94 (t, J = 7.8 Hz, 3H), 1.38 (m, J = 7.5 Hz, 2H), 1.64 (m, 4H), 1.85
(m, 2H), 1.95 (m, 2H), 2.68 (t, J = 7.5 Hz, 2H), 3.46 (t, J = 6.6 Hz,
2H), 4.05 (t, J = 6.6 Hz, 2H), 6.99 (d, J = 8.7 Hz, 2H), 7.30 (d, J =
8.2 Hz, 2H), 7.80 (d, J = 8.2 Hz, 2H), 7.89 ppm (d, J = 8.7 Hz, 2H); 13C
NMR (75.5 MHz, CDCl3): d = 13.9, 22.3, 24.8, 28.4, 32.4, 33.5, 33.6, 35.5,
67.8, 114.6, 122.5, 124.5, 129.0, 145.8, 147.0, 151.0, 161.2 ppm; UV/Vis
(PhMe): lmax = 352 nm; EI-MS: m/z: 403 [M]+, 405 [M+2]+ ; elemental
analysis calcd for C21H27BrN2O: C 62.66, H 6.76, N 6.96; found: C 62.34,
H 6.75, N 6.64.
AzoG0:
A
mixture of tris-5-bromopentyl phloroglucinol[8] (0.10 g,
0.17 mmol), 4-[(4-butylphenyl)azo]phenol (0.22 g, 0.87 mmol), K2CO3
(0.10 g, 0.72 mmol), and 18-C-6 (cat.) in MeCN (20 mL) was refluxed for
17 h, filtered, and the solvents were removed in vacuo. Flash chromatog-
raphy of the resulting residue (SiO2, PhMe/EtOAc 98:2) afforded AzoG0
as an orange solid (0.162 g, 85%). Rf = 0.80 (PhMe/EtOAc 98:2); m.p.
70 738C; 1H NMR (300 MHz, CDCl3): d = 0.94 (t, J = 7.5 Hz, 9H),
1.38 (m, J = 7.2 Hz, 6H), 1.63 (m, 12H), 1.86 (m, 12H), 2.67 (t, J =
7.2 Hz, 6H), 3.96 (t, J = 6.3 Hz, 6H), 4.06 (t, J = 6.3 Hz, 6H), 6.08 (s,
3H), 6.99 (d, J = 9.0 Hz, 6H), 7.30 (d, J = 8.1 Hz, 6H), 7.79 (d, J =
Conclusion
The synthesis and study of azobenzene-functionalized poly(-
alkyl aryl ether) dendrimers is an interesting extension of a
number of previous studies on azobenzene-containing den-
drimers. The major findings from our detailed studies are:
8.1 Hz, 6H), 7.89 ppm (d,
J =
9.0 Hz, 6H); 13C NMR (75.5 MHz,
CDCl3): d = 13.9, 22.3, 22.7, 28.9, 33.5, 35.5, 67.8, 68.1, 93.9, 114.7, 122.5,
124.6, 129.0, 145.8, 147.0, 151.0, 160.9, 161.4 ppm; UV/Vis (PhMe): lmax
= 350 nm; MALDI-TOF MS: m/z: 1094.4 [M+H]+ ; elemental analysis
calcd for C69H84N6O6: C 75.79, H 7.74, N 7.68; found: C 75.73, H 8.22, N
6.86.
AzoG1: A mixture of G1-(OH)6[8] (0.040 g, 0.06 mmol), 4-[(4-butylphenyl)-
azo]phenol (0.228 g, 0.56 mmol), K2CO3 (0.080 g, 0.56 mmol), and
[18]crown-6 (cat.) in MeCN (12 mL) was refluxed for 42 h, filtered, and
the solvents were removed in vacuo. Flash chromatography of the crude
residue (SiO2, PhMe/EtOAc 98:2) afforded AZoG1 as an orange, low-
1) the dendritic architecture influences and promotes prop-
erties of azobenzene that are normally associated only
with aggregates of azobenzene; 2) the anomalous solid-
state fluorescence emission of the dendrimers AzoG1--
AzoG3 at room temperature are characteristic, reflecting
the optimum levels of inherent structural features that
facilitate aggregate formation; and 3) the liquid crystal-
line AzoM is also fluorescent in its liquid crystalline and
696
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Chem. Eur. J. 2004, 10, 689 698

Poly(alkyl aryl ether) Dendrimers
689 698
melting solid (0.12 g, 80%). Rf = 0.53 (PhMe/EtOAc 98:2); m.p. 40
428C; 1H NMR (300 MHz, CDCl3): d = 0.93 (t, J = 7.2 Hz, 18H), 1.37
(m, J = 7.5 Hz, 12H), 1.61 (br, 30H), 1.83 (br, 36H), 2.66 (t, J = 7.5 Hz,
12H), 3.93 (br, 24H), 4.03 (t, J = 6.0 Hz, 12H), 6.06 (s, 12H), 6.97 (d, J
= 7.8 Hz, 12H), 7.28 (d, J = 7.5 Hz, 12H), 7.78 (d, J = 7.5 Hz, 12H),
7.87 ppm (d, J = 7.8 Hz, 12H); 13C NMR (75.5 MHz, CDCl3): d = 13.9,
22.3, 22.7, 28.9, 29.0, 33.5, 35.5, 67.7, 67.8, 68.1, 93.9, 114.7, 122.5, 124.5,
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129.0, 145.8, 147.0, 151.0, 160.9, 161.3 ppm; UV/Vis (PhMe): lmax
=
352 nm; MALDI-TOF MS: m/z: 2643.1 [M]+ ; elemental analysis calcd
for C165H204N12O18: C 74.97, H 7.78, N 6.36; found: C 75.16, H 8.02, N
6.11.
[8]
AzoG2: A mixture of G2-(OH)12 (0.14 g, 0.075 mmol), 4-[(4-butylphe-
nyl)azo]phenol (0.47 g, 1.17 mmol), and Cs2CO3 (0.38 g, 1.17 mmol) in
DMF (8 mL) was stirred over a period of nine days. The reaction mixture
was then filtered, the filtrate diluted in CH2Cl2, washed with water and
brine, dried, and concentrated. Flash chromatographic purification (SiO2,
PhMe/EtOAc 99:1) of the resulting product afforded AzoG2 as an
orange, glassy solid (0.26 g, 61%). Rf = 0.33 (PhMe/EtOAc 98:2); 1H
NMR (300 MHz, CDCl3): d = 0.93 (t, J = 7.2 Hz, 36H), 1.34 (m, J =
7.2 Hz, 24H), 1.61 (br, 66H), 1.83 (br, 84H), 2.66 (t, J = 7.2 Hz, 24H),
3.92 (br, 60H), 4.02 (brt, J = 6.3 Hz, 24H), 6.06 (s, 30H), 6.97 (d, J =
9.0 Hz, 24H), 7.28 (d, J = 8.4 Hz, 24H), 7.79 (d, J = 8.4 Hz, 24H),
7.87 ppm (d, J = 9.0 Hz, 24H); 13C NMR (75.5 MHz, CDCl3): d = 13.9,
22.3, 22.7, 28.9, 29.0, 33.4, 35.5, 67.7, 68.0, 93.8, 114.6, 122.5, 124.5, 129.0,
145.7, 147.0, 151.0, 160.9, 161.3 ppm; UV/Vis (PhMe): lmax = 352 nm;
MALDI-TOF MS: m/z: 5738.9 [M]+
; elemental analysis cacld for
C357H444N24O42: C 74.66, H 7.79, N 5.85; found: C 75.24., H 8.07, N 5.70.
[8]
AzoG3: A mixture of G3-(OH)24 (0.07 g, 0.017 mmol), 4-[(4-butylphe-
nyl)azo]phenol (0.23 g, 0.56 mmol), and Cs2CO3 (0.18 g, 0.56 mmol) in
DMF (5 mL) was stirred over a period of nine days. The reaction mixture
was then filtered, the was filtrate diluted in CH2Cl2 and washed with
water and brine, dried, and concentrated. Flash chromatographic purifi-
cation (SiO2, PhMe/EtOAc 98:2) of the resulting product afforded
AZoG3 as an orange, glassy solid (0.08 g, 40%). Rf
= 0.58 (PhMe/
EtOAc 95:5); 1H NMR (300 MHz, CDCl3): d = 0.93 (t, J = 7.2 Hz,
72H), 1.36 (m, J = 7.5 Hz, 48H), 1.62 (br, 138H), 1.81(br, 180H), 2.65
(t, J = 7.2 Hz, 48H), 3.90 (br, 132H), 4.01 (br, 48H), 6.04 (br, 66H),
6.96 (d, J = 8.4 Hz, 48H), 7.28 (d, J = 8.0 Hz, 48H), 7.78 (d, J =
8.0 Hz, 48H), 7.87 ppm (d, J
=
8.4 Hz, 48H); 13C NMR (75.5 MHz,
CDCl3): d = 13.9, 22.3, 22.7, 28.9, 33.4, 35.5, 67.7, 68.0, 93.8, 114.6, 122.5,
124.5, 129.0, 145.7, 146.9, 150.9, 160.8, 161.3 ppm; UV/Vis (PhMe): lmax
= 352 nm; elemental analysis calcd for C741H924N48O90: C 74.52, H 7.80,
N 5.63; found: C 73.90, H 7.75, N 5.23.
Instrumentation: Phase transitions were observed using a Nikon HFX
35A Optiphot-2 polarized light optical microscope, equipped with a
Linkam THMS 600 heating and freezing stage connected to a Linkam
TP92 temperature programmer. DSC analyses were preformed under air,
using a DuPont DSC 2010 differential scanning calorimeter attached to a
Thermal Analyst 2100 data station. Absorption spectra were recorded on
a Shimadzu 3101PC UV/Vis-NIR spectrophotometer. Steady-state pho-
tolysis was carried out using a 200 W high-pressure mercury lamp, in
combination with a 360 nm Oriel bandpass filter. Laser photolysis was
carried out using the third harmonic (355 nm, 5 mJ per pulse, pulse width
10 ns) of a Quanta Ray GCR-12 Nd:YAG laser. The excitation and emis-
sion spectra were measured on a SPEX Fluorolog F112X spectrofluorim-
eter.
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N. Jayaraman, S. Das et al.
FULL PAPER
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Received: July 2, 2003
Revised: September 15, 2003 [F5297]
698
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2004, 10, 689 698